Solar cell

ABSTRACT

Discussed is a solar cell including a first conductive region positioned at a front surface of a semiconductor substrate and containing impurities of a first conductivity type or a second conductivity type, a second conductive region positioned at a back surface of the semiconductor substrate and containing impurities of a conductivity type opposite a conductivity type of impurities of the first conductive region, a first electrode positioned on the front surface of the semiconductor substrate and connected to the first conductive region, and a second electrode positioned on the back surface of the semiconductor substrate and connected to the second conductive region. Each of the first and second electrodes includes metal particles and a glass frit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.15/889,950 filed on Feb. 6, 2018, which claims the priority benefit ofKorean Patent Application No. 10-2017-0016816 filed on Feb. 7, 2017,Korean Patent Application No. 10-2017-0044075 filed on Apr. 5, 2017, andKorean Patent Application No. 10-2017-0160457 filed Nov. 28, 2017, theentire contents of all of these applications are hereby expresslyincorporated by reference in their entirety into the presentapplication.

BACKGROUND OF THE INVENTION Field of the Invention

Embodiments of the invention relate to a solar cell.

Description of the Related Art

Recently, as existing energy sources such as petroleum and coal areexpected to be depleted, interests in alternative energy sources forreplacing the existing energy sources are increasing. Among thealternative energy sources, solar cells for generating electric energyfrom solar energy are attracting attention because they are rich inenergy resources and have no problem of environmental pollution.

A solar cell generally includes a substrate and an emitter region formedof semiconductors which respectively have different conductivity types,for example, a p-type and an n-type, and electrodes respectivelyconnected to the substrate and the emitter region of the differentconductivity types. In this instance, the substrate and the emitterregion form a p-n junction.

When light is incident on the solar cell, a plurality of electron-holepairs are produced in the semiconductors and are separated intoelectrons and holes by the incident light. The electrons move to then-type semiconductor, for example, the emitter region, and the holesmove to the p-type semiconductor, for example, the substrate. Then, theelectrons and the holes are collected by the electrodes electricallyconnected to the substrate and the emitter region. The electrodes areconnected to each other using electric wires to thereby obtain electricpower.

A solar cell including a conductive region formed by doping a backsurface of a semiconductor substrate with impurities and a passivationlayer between the conductive region and the semiconductor substrate isrecently under development to improve an open-circuit voltage Voc.

However, because a thickness of the conductive region in the solar cellhaving the above-described structure is greatly less than a thicknessaccording to a related art, a back electrode is short-circuited with thesemiconductor substrate because metal particles included in the backelectrode penetrate the passivation layer between the conductive regionand the semiconductor substrate when the back electrode connected to theconductive region is formed. Hence, there is a problem that a defect ofthe solar cell is caused.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a solar cell having a structurecapable of reducing a defect of the solar cell while improving anopen-circuit voltage.

In one aspect, there is provided a solar cell including a semiconductorsubstrate; a first conductive region positioned at a front surface ofthe semiconductor substrate, the first conductive region containingimpurities of a first conductivity type or impurities of a secondconductivity type; a second conductive region positioned at a backsurface of the semiconductor substrate, the second conductive regioncontaining impurities of a conductivity type opposite a conductivitytype of impurities contained in the first conductive region, and thesecond conductive region including a silicon material; a first electrodepositioned on the front surface of the semiconductor substrate andconnected to the first conductive region; and a second electrodepositioned on the back surface of the semiconductor substrate andconnected to the second conductive region, wherein each of the first andsecond electrodes includes metal particles and a glass frit, and whereina content of the glass frit per unit volume contained in the secondelectrode is less than a content of the glass frit per unit volumecontained in the first electrode.

For example, the content of the glass frit per unit volume contained inthe first electrode may be 6 wt % to 8 wt %, and the content of theglass frit per unit volume contained in the second electrode may be 2.5wt % to 5.0 wt %.

A content of the metal particles per unit volume contained in the firstelectrode may be more than a content of the metal particles per unitvolume contained in the second electrode. For example, the content ofthe metal particles per unit volume contained in the first electrode maybe 82 wt % to 92 wt %, and the content of the metal particles per unitvolume contained in the second electrode may be 68 wt % to 73 wt %.

The solar cell may further include an anti-reflection layer positionedon a front surface of the first conductive region, a control passivationlayer positioned between the back surface of the semiconductor substrateand the second conductive region, the control passivation layerincluding a dielectric material, and a back passivation layer positionedon a back surface of the second conductive region. A thickness of theback passivation layer may be less than a thickness of theanti-reflection layer and may be greater than a thickness of the controlpassivation layer.

For example, the thickness of the anti-reflection layer is 100 nm to 140nm, and the thickness of the back passivation layer may be 65 nm to 105nm within a range that is less than the thickness of the anti-reflectionlayer.

The thickness of the control passivation layer may be less than thethickness of the back passivation layer and may be, for example, 0.5 nmto 10 nm.

Further, a thickness of the second conductive region may be less than athickness of the first conductive region. For example, the thickness ofthe first conductive region may be 300 nm to 700 nm, and the thicknessof the second conductive region may be 290 nm to 390 nm within a rangethat is less than the thickness of the first conductive region.

The glass frit of the second electrode may include at least one of aPbO-based material and a BiO-based material.

The glass frit of the second electrode may further include telluriumoxide (TeO).

A melting point of the glass frit including tellurium oxide (TeO) may be200° C. to 500° C.

The second electrode may include a first layer where the glass fritincluding tellurium oxide (TeO) is positioned at an interface betweenthe second electrode and the second conductive region, and a secondlayer where the metal particles and the glass frit not includingtellurium oxide (TeO) is positioned on the first layer.

Further, crystallites formed by combining the metal particles of thesecond electrode and silicon of the second conductive region may bedistributed at an interface between the first layer and the secondconductive region.

The glass frit of the first electrode may include at least one of aPbO-based material and a BiO-based material. The glass frit of the firstelectrode may further include tellurium oxide (TeO).

The metal particles of the first electrode may include first metalparticles having a circular shape or an oval shape and second metalparticles which have a long axis and have a plate shape having an unevensurface. The metal particles of the second electrode may include thefirst metal particles and may not include the second metal particles.

A length of the long axis of the second metal particle included in thefirst electrode may be greater than a size of the first metal particleincluded in each of the first and second electrodes.

In another aspect, there is provided a solar cell including asemiconductor substrate; a first conductive region positioned at a frontsurface of the semiconductor substrate, the first conductive regioncontaining impurities of a first conductivity type or impurities of asecond conductivity type; a control passivation layer positioned on aback surface of the semiconductor substrate, the control passivationlayer including a dielectric material; a second conductive regionpositioned at the back surface of the semiconductor substrate, thesecond conductive region containing impurities of a conductivity typeopposite a conductivity type of impurities contained in the firstconductive region, and the second conductive region including apolycrystalline silicon material; a first electrode positioned on thefront surface of the semiconductor substrate and connected to the firstconductive region; and a second electrode positioned on the back surfaceof the semiconductor substrate and connected to the second conductiveregion, wherein each of the first and second electrodes includes metalparticles and a glass frit, and wherein the glass frit of the firstelectrode includes tellurium oxide (TeO).

The glass frit of the second electrode may include tellurium oxide(TeO).

Embodiments of the invention allow a content of the glass frit containedin the second electrode positioned on the back surface of thesemiconductor substrate to be less than a content of the glass fritcontained in the first electrode positioned on the front surface of thesemiconductor substrate and thus can control a depth to which the secondelectrode is fired through when the second electrode is connected to thesecond conductive region through a thermal process.

Accordingly, embodiments of the invention can prevent the metalparticles of the second electrode penetrating the second conductiveregion and the control passivation layer from being short-circuited withthe semiconductor substrate and prevent a defect that may occur in amanufacturing process while increasing an open-circuit voltage Voc ofthe solar cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a partial perspective view of a solar cell according to anembodiment of the invention.

FIG. 2 is a cross-sectional view of a solar cell shown in FIG. 1 .

FIG. 3 is a table indicating an experimental result of a contactresistance, a passivation function (or a recombination level), and anopen-circuit voltage depending on a content of a glass frit included ina second electrode.

FIG. 4 is an enlarged cross-sectional view of a portion of a solar cellincluding a semiconductor substrate, a control passivation layer, asecond conductive region, and a second electrode when a content of aglass frit of FIG. 3 exceeds an appropriate level.

FIG. 5 is an enlarged cross-sectional view of a portion of a solar cellincluding a semiconductor substrate, a control passivation layer, asecond conductive region, and a second electrode when a content of aglass frit of FIG. 3 is at an appropriate level.

FIG. 6 is an enlarged cross-sectional view of a portion of a solar cellincluding a semiconductor substrate, a control passivation layer, asecond conductive region, and a second electrode when a glass fritfurther includes tellurium oxide (TeO) in a state where a content of theglass frit of FIGS. 3 and 4 maintains an appropriate level.

FIG. 7 illustrates metal particles included in first and secondelectrodes according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to embodiments of the invention,examples of which are illustrated in the accompanying drawings. Thisinvention may, however, be embodied in many different forms and shouldnot be construed as limited to the embodiments set forth herein.Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts. It will be noted that adetailed description of known arts will be omitted if it is determinedthat the detailed description of the known arts can obscure theembodiments of the invention.

Reference will now be made in detail to embodiments of the invention,examples of which are illustrated in the accompanying drawings. Thisinvention may, however, be embodied in many different forms and shouldnot be construed as limited to the embodiments set forth herein.Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts. It will be noted that adetailed description of known arts will be omitted if it is determinedthat the detailed description of the known arts can obscure theembodiments of the invention.

In the following description, a content of metal particles and a contentof a glass frit indicate a content per unit volume unless otherwisespecified.

Embodiments of the invention will be described with reference to FIGS. 1to 7 .

FIG. 1 is a partial perspective view of a solar cell according to anembodiment of the invention. FIG. 2 is a cross-sectional view of a solarcell shown in FIG. 1 .

As shown in FIG. 1 , an example of a solar cell according to anembodiment of the invention may include a semiconductor substrate 110, afirst conductive region 120, an anti-reflection layer 130, a controlpassivation layer 160, a second conductive region 170, a backpassivation layer 190, a first electrode 140, and a second electrode150.

FIG. 1 illustrates that the solar cell according to the embodiment ofthe invention includes the anti-reflection layer 130, by way of example.However, embodiments are not limited thereto. For example, theanti-reflection layer 130 may be omitted, if desired or necessary.However, when the solar cell includes the anti-reflection layer 130,efficiency of the solar cell can be further improved. Thus, theembodiment of the invention is described using the solar cell includingthe anti-reflection layer 130 as an example.

The semiconductor substrate 110 may be formed of at least one of singlecrystal silicon and polycrystalline silicon each containing impuritiesof a first conductivity type or a second conductivity type. For example,the semiconductor substrate 110 may be formed of a single crystalsilicon wafer.

The semiconductor substrate 110 may include impurities of the firstconductivity type or impurities of the second conductivity type. Inembodiments disclosed herein, impurities of the first conductivity typemay be impurities of an n-type or a p-type, and impurities of the secondconductivity type may be impurities of a conductivity type opposite thefirst conductivity type.

For example, when the first conductivity type is the p-type, the secondconductivity type may be the n-type. On the contrary, when the firstconductivity type is the n-type, the second conductivity type may be thep-type.

In the following description, an embodiment in which the firstconductivity type is the p-type, the second conductivity type is then-type, and the semiconductor substrate 110 contains impurities of thesecond conductivity type, i.e., n-type impurities will be described asan example.

When the semiconductor substrate 110 is of the p-type, the semiconductorsubstrate 110 may be doped with impurities of a group III element suchas boron (B), gallium (Ga), and indium (In). Alternatively, when thesemiconductor substrate 110 is of the n-type, the semiconductorsubstrate 110 may be doped with impurities of a group V element such asphosphorus (P), arsenic (As), and antimony (Sb).

In the following description, embodiments of the invention are describedusing an example where impurities contained in the semiconductorsubstrate 110 are impurities of the second conductivity type and aren-type impurities. However, embodiments of the invention are not limitedthereto.

A front surface and a back surface of the semiconductor substrate 110may be an uneven surface having a plurality of texturing uneven portionsor having uneven characteristics. Thus, the first conductive region 120positioned at the front surface of the semiconductor substrate 110 mayhave an uneven surface, and the second conductive region 170 positionedat the back surface of the semiconductor substrate 110 may have anuneven surface.

In embodiments disclosed herein, “texturing uneven portion” indicates anuneven portion formed on the surface of the solar cell in order toreduce an amount of reflected light and may have, for example, a pyramidshape.

Hence, an amount of light reflected from the front surface of thesemiconductor substrate 110 can decrease, and an amount of lightincident on the inside of the semiconductor substrate 110 can increase.

The first conductive region 120 is positioned at the front surface ofthe semiconductor substrate 110 on which light is incident, and maycontain impurities of the first conductivity type or the secondconductivity type.

Thus, the first conductive region 120 may contain n-type impurities orp-type impurities.

For example, when the semiconductor substrate 110 contains n-typeimpurities, the first conductive region 120 may contain p-typeimpurities and form a p-n junction together with the semiconductorsubstrate 110. In this instance, the first conductive region 120 mayserve as an emitter region.

On the contrary, when the semiconductor substrate 110 contains p-typeimpurities, the first conductive region 120 may contain n-typeimpurities at a higher concentration than the semiconductor substrate110 and may serve as a front surface field region.

Alternatively, when the semiconductor substrate 110 contains p-typeimpurities, the first conductive region 120 may contain n-type or p-typeimpurities. When the first conductive region 120 is of an n-type,impurities of a group V element such as phosphorus (P), arsenic (As),and antimony (Sb) may be distributed into the front surface of thesemiconductor substrate 110 through a thermal process to form the firstconductive region 120.

On the contrary, when the first conductive region 120 is of a p-type,impurities of a group III element such as boron (B), gallium (Ga), andindium (In) may be distributed into the front surface of thesemiconductor substrate 110 through the thermal process to form thefirst conductive region 120.

In the following description, embodiments of the invention are describedusing an example where the first conductive region 120 containsimpurities of a conductivity type opposite a conductivity type ofimpurities contained in the semiconductor substrate 110 to serve as anemitter region.

Because the first conductive region 120 is formed by distributing n-typeor p-type impurities into the front surface of the semiconductorsubstrate 110 as described above, the first conductive region 120 may beformed of single crystal silicon material or polycrystalline siliconmaterial that is the same as the semiconductor substrate 110.

Thus, when the semiconductor substrate 110 is formed of a single crystalsilicon wafer, the first conductive region 120 may be formed of a singlecrystal silicon wafer. When the semiconductor substrate 110 is formed ofa polycrystalline silicon wafer, the first conductive region 120 may beformed of a polycrystalline silicon wafer.

A thickness of the first conductive region 120 may be 300 nm to 700 nm.

The anti-reflection layer 130 is positioned on the first conductiveregion 120. The anti-reflection layer 130 may be formed of at least oneof aluminum oxide (AlOx), silicon nitride (SiNx), silicon oxide (SiOx),and silicon oxynitride (SiOxNy) and may be formed as a single layer or aplurality of layers.

The anti-reflection layer 130 can reduce a reflectance of light incidenton the solar cell and increase selectivity of a predetermined wavelengthband, thereby increasing the efficiency of the solar cell.

A thickness of the anti-reflection layer 130 may be 100 nm to 140 nm.

The first electrode 140 is disposed on the first conductive region 120and directly contacts the first conductive region 120. The firstelectrode 140 may be electrically connected to the first conductiveregion 120.

As shown in FIGS. 1 and 2 , the first electrode 140 may include aplurality of first finger electrodes 141 extended in a first direction x(for example, x-axis direction) and a plurality of first bus barelectrodes 142 that is extended in a second direction y (for example,y-axis direction) intersecting the plurality of first finger electrodes141 and connects the plurality of first finger electrodes 141.

As shown in FIGS. 1 and 2 , when the semiconductor substrate 110 is ofthe n-type, the first electrode 140 may collect carriers (e.g., holes)moving to the p-type first conductive region 120.

The first electrode 140 is connected to an interconnector (not shown)connecting solar cells to each other and outputs collected carriers toan external device.

The first electrode 140 may include metal particles and a glass frit.More specifically, the metal particle may include at least one of nickel(Ni), copper (Cu), silver (Ag), aluminum (Al), tin (Sn), zinc (Zn),indium (In), titanium (Ti), gold (Au), and a combination thereof.

A melting point of the metal particle may be higher than a melting pointof the glass frit. Thus, after the first electrode 140 is completed, themetal particles may maintain an original shape that they had in a pastestate, and the glass frit may be fired after it is completely melted.Therefore, the glass frit may have a different shape from a shape thatit had in a paste state.

The first electrode 140 may be formed by patterning a first electrodepaste on the anti-reflection layer 130 in a state where theanti-reflection layer 130 is formed on a front surface of the firstconductive region 120, causing the first electrode paste to penetratethe anti-reflection layer 130 while the first electrode paste is firedthrough the anti-reflection layer 130 through a thermal process, andfiring the first electrode paste in a state where the first electrodepaste is electrically connected to the first conductive region 120.

The control passivation layer 160 is entirely positioned on the backsurface of the semiconductor substrate 110 and may include a dielectricmaterial.

For example, as shown in FIGS. 1 and 2 , the control passivation layer160 on the back surface of the semiconductor substrate 110 may directlycontact the back surface of the semiconductor substrate 110.

Further, the control passivation layer 160 may be formed on the entireback surface except an edge of the back surface of the semiconductorsubstrate 110.

The control passivation layer 160 can perform a dopant control functionor a diffusion barrier that prevents a dopant of the second conductiveregion 170 from being excessively diffused into the semiconductorsubstrate 110. In addition, the control passivation layer 160 canperform a passivation function of the back surface of the semiconductorsubstrate 110.

The control passivation layer 160 may include various materials that cancontrol the diffusion of the dopant and transfer multiple carriers. Forexample, the control passivation layer 160 may include oxide, nitride,semiconductor, conducting polymer, or the like.

For example, the control passivation layer 160 may be a silicon oxidelayer including silicon oxide. This is because the silicon oxide layerhas good passivation characteristics and smoothly transfers carriers.

Further, the silicon oxide layer may be easily formed on the surface ofthe semiconductor substrate 110 by various processes.

The control passivation layer 160 may be formed by various methods suchas vapor deposition, thermal oxidation, and chemical oxidation. However,the control passivation layer 160 may be omitted, if necessary ordesired.

A thickness of the control passivation layer 160 may be less than athickness of the back passivation layer 190 and may be, for example, 0.5nm to 10 nm. The control passivation layer 160 may be formed by anoxidation process, a low pressure chemical vapor deposition (LPCVD)process, or a plasma-enhanced chemical vapor deposition (PECVD) process.

A reason why the thickness of the control passivation layer 160 islimited to 0.5 nm to 10 nm is to implement a tunneling effect. Thecontrol passivation layer 160 can perform a portion of a passivationfunction on the back surface of the semiconductor substrate 110.

The second conductive region 170 is positioned at the back surface ofthe semiconductor substrate 110. The second conductive region 170 maycontain impurities of a conductivity type opposite a conductivity typeof impurities contained in the first conductive region 120 and may beformed of a polycrystalline silicon material.

Thus, when the first conductive region 120 serves as the emitter region,the second conductive region 170 may serve as a back surface fieldregion.

As shown in FIGS. 1 and 2 , the second conductive region 170 may bepositioned at a back surface of the control passivation layer 160 andspaced apart from the semiconductor substrate 110.

The second conductive region 170 may be formed on the controlpassivation layer 160 using a chemical vapor deposition (CVD) method.For example, the second conductive region 170 may be formed bydepositing a polycrystalline silicon material containing impurities ofthe first conductivity type on the control passivation layer 160.Alternatively, the second conductive region 170 may be formed bydepositing an amorphous silicon material containing impurities of thefirst conductivity type on the control passivation layer 160 and thencrystallizing the amorphous silicon material into a polycrystallinesilicon material through a thermal process.

Hence, as shown in FIGS. 1 and 2 , the second conductive region 170 isnot formed inside the semiconductor substrate 110 and is formed on theback surface of the semiconductor substrate 110 so that it does notdirectly contact the semiconductor substrate 110 and is spaced apartfrom the semiconductor substrate 110. As described above, when thesecond conductive region 170 is formed on the back surface of thecontrol passivation layer 160, an open-circuit voltage Voc of the solarcell can be further improved.

Because the second conductive region 170 is not formed inside thesemiconductor substrate 110 and is formed outside the semiconductorsubstrate 110, the thermal processing of the semiconductor substrate 110can be minimized in a process for forming the second conductive region170. Hence, a reduction in characteristics of the semiconductorsubstrate 110 can be prevented, and the efficiency of the solar cellaccording to the embodiment of the invention can be further improved.

A thickness of the second conductive region 170 may be determined inconsideration of a deposition time of the second conductive region 170and also selected as an appropriate thickness capable of sufficientlyperforming a function of the second conductive region 170. For example,the thickness of the second conductive region 170 may be 290 nm to 390nm within a range that is less than the thickness of the firstconductive region 120.

The second electrode 150 is disposed on the second conductive region 170and directly contacts the second conductive region 170. The secondelectrode 150 may be electrically connected to the second conductiveregion 170.

As shown in FIGS. 1 and 2 , the second electrode 150 may include aplurality of second finger electrodes 151 extended in the firstdirection x and a plurality of second bus bar electrodes 152 that isextended in the second direction y intersecting the plurality of secondfinger electrodes 151 and connects the plurality of second fingerelectrodes 151.

The second electrode 150 is connected to an interconnector connectingsolar cells to each other and outputs collected carriers to the externaldevice.

The second electrode 150 may include metal particles and a glass frit.More specifically, the metal particle having a melting point higher thana melting point of the glass frit may include at least one of nickel(Ni), copper (Cu), silver (Ag), aluminum (Al), tin (Sn), zinc (Zn),indium (In), titanium (Ti), gold (Au), and a combination thereof.

The second electrode 150 may be formed by patterning a second electrodepaste on a back surface of the back passivation layer 190 in a statewhere the back passivation layer 190 is formed on a back surface of thesecond conductive region 170, causing the second electrode paste topenetrate the back passivation layer 190 while the second electrodepaste is fired through the back passivation layer 190 through a thermalprocess, and firing the second electrode paste in a state where thesecond electrode paste is electrically connected to the secondconductive region 170.

As shown in FIGS. 1 and 2 , the back passivation layer 190 may bepositioned on a remaining portion excluding a formation portion of thesecond electrode 150 from the back surface of the second conductiveregion 170.

The back passivation layer 190 may include a dielectric material and maybe formed as a single layer or a plurality of layers. The backpassivation layer 190 may have specific fixed carriers in considerationof a conductivity type of the second conductive region 170.

The back passivation layer 190 may be formed of at least one of siliconcarbide (SiC), silicon oxide (SiOx), silicon nitride (SiNx),hydrogenated SiNx, aluminum oxide (AlOx), silicon oxynitride (SiON), orhydrogenated SiON.

The back passivation layer 190 may perform a passivation function of theback surface of the second conductive region 170.

The thickness of the back passivation layer 190 may be greater than thethickness of the control passivation layer 160 and may be less than thethickness of the anti-reflection layer 130, in order to sufficientlyperform the passivation function of the back surface of the secondconductive region 170.

Thus, the thickness of the back passivation layer 190 may be, forexample, 65 nm to 105 nm within a range that is greater than thethickness of the control passivation layer 160 and is less than thethickness of the anti-reflection layer 130.

So far, the embodiment of the invention described that the firstconductive region 120 serves as the emitter region and the secondconductive region 170 serves as the back surface field region, by way ofexample.

However, embodiments are not limited thereto. For example, thesemiconductor substrate 110 may contain p-type impurities, the firstconductive region 120 may contain p-type impurities and serve as a frontsurface field region, and the second conductive region 170 may containn-type impurities and serve as a back surface field region

In the above-described solar cell, each of the first and secondelectrodes 140 and 150 may include the metal particles and the glassfrit.

In embodiments disclosed herein, the metal particles may be, forexample, Ag particles and may be related to the conductivity of each ofthe first and second electrodes 140 and 150. The glass frit may berelated to a depth to which each of the first and second electrodes 140and 150 is fired through.

The glass frit used in the first and second electrodes 140 and 150according to the embodiment of the invention may include at least one ofPbO-based material or BiO-based material.

A content of the metal particles and a content of the glass fritincluded in each of the first and second electrodes 140 and 150 may bedifferently determined in consideration of the materials and thethicknesses of the first conductive region 120 and the second conductiveregion 170.

For example, a content of the metal particles per unit volume containedin the first electrode 140 may be larger than a content of the metalparticles per unit volume contained in the second electrode 150.

The first electrode 140 collects holes having a relatively slow movingspeed, and a linewidth of the first electrode 140 has to be smaller thana linewidth of the second electrode 150 so that a larger amount of lightis received. Therefore, the first electrode 140 may be necessary to havethe conductivity relatively greater than the conductivity of the secondelectrode 150.

To this end, the content of the metal particles per unit volumecontained in the first electrode 140 may be larger than the content ofthe metal particles per unit volume contained in the second electrode150.

For example, the content of the metal particles per unit volumecontained in the first electrode 140 may be 82 wt % to 92 wt %, and thecontent of the metal particles per unit volume contained in the secondelectrode 150 may be 68 wt % 73 wt %.

As described above with reference to FIGS. 1 and 2 , in the structure ofthe solar cell according to the embodiment of the invention, the firstconductive region 120 may be formed of, for example, a single crystalsilicon substrate that is the same as the semiconductor substrate 110;the second conductive region 170 may be formed of a polycrystallinesilicon material; the thickness of the second conductive region 170 maybe relatively less than the thickness of the first conductive region120; and the thickness of the back passivation layer 190 may be lessthan the thickness of the anti-reflection layer 130.

When the second electrode paste containing a glass frit having the samecontent as a content of a glass frit contained in the first electrodepaste is patterned on the control passivation layer 160 that is verythin or relatively thin, the second conductive region 170, and the backpassivation layer 190 and is thermally processed, the second electrodepaste may penetrate the control passivation layer 160 as well as theback passivation layer 190 and the second conductive region 170 duringthe thermal process, may be directly electrically connected to thesemiconductor substrate 110, and may be short-circuited with thesemiconductor substrate 110.

Thus, the embodiment of the invention can allow a content of a glassfrit contained in the second electrode 150 to be different from acontent of a glass frit contained in the first electrode 140.

More specifically, the embodiment of the invention can allow a contentof a glass frit per unit volume contained in the second electrode 150 tobe less than a content of a glass frit per unit volume contained in thefirst electrode 140, in order to control a depth to which the secondelectrode paste penetrating the back passivation layer 190 is firedthrough toward the second conductive region 170.

Thus, the depth to which the second electrode paste is fired through canbe controlled by adjusting the content of the glass frit per unitvolume.

More specifically, for example, when a content of a glass frit per unitvolume in the first electrode 140 is 6 wt % to 8 wt %, a content of aglass frit per unit volume in the second electrode 150 may be 2.5 wt %to 5.0 wt %.

When the content of the glass frit per unit volume in the secondelectrode 150 is 2.5 wt % to 5.0 wt % as described above, the embodimentof the invention can maintain a contact resistance between the secondelectrode 150 and the second conductive region 170 at a sufficiently lowlevel and prevent the second electrode 150, that penetrates the controlpassivation layer 160 as well as the back passivation layer 190 and thesecond conductive region 170, from being short-circuited with thesemiconductor substrate 110. Further, the embodiment of the inventioncan prevent a recombination, which may occur in the back surface of thesemiconductor substrate 110, due to the metal particles contained in thesecond electrode 150 and can allow the control passivation layer 160 tosufficiently perform the passivation function. In addition, theembodiment of the invention can prevent a damage of the controlpassivation layer 160 and improve the open-circuit voltage Voc of thesolar cell at a good level.

An effect depending on a content of a glass frit of the second electrode150 will be described in detail below with reference to FIGS. 3, 4, and5 .

FIG. 3 is a table indicating an experimental result of a contactresistance, a passivation function (or a recombination level), and anopen-circuit voltage depending on a content of a glass frit included inthe second electrode 150.

FIG. 4 is an enlarged cross-sectional view Si of FIG. 2 of a portion ofthe solar cell including the semiconductor substrate 110, the controlpassivation layer 160, the second conductive region 170, and the secondelectrode 150 when a content of the glass frit of FIG. 3 exceeds anappropriate level.

FIG. 5 is an enlarged cross-sectional view Si of FIG. 2 of a portion ofthe solar cell including the semiconductor substrate 110, the controlpassivation layer 160, the second conductive region 170, and the secondelectrode 150 when a content of the glass frit of FIG. 3 is at theappropriate level.

In the table illustrated in FIG. 3 , a content of a glass frit of thesecond electrode 150 indicates a content of a glass frit per unit volumeand may be different from a content of a glass frit 150G per unit volumeincluded in the second electrode paste.

This is because the second electrode paste before a thermal process mayfurther include a binder of a resin material and a solvent in additionto metal particles 150M and the glass frit 150G, and the binder and thesolvent may be mostly oxidized or evaporated during the thermal process.

Hence, as shown in FIGS. 4 and 5 , after the thermal process, the metalparticles 150M and the glass frit 150G may be present in the secondelectrode 150.

Thus, a content of the glass frit 150G of the second electrode paste maybe different from a content of the glass frit 150G of the secondelectrode 150 that is fired after the thermal process. For example, thecontent of the glass frit 150G may increase by about 0.5 wt % to 1.0 wt%, as compared to before the second electrode 150 is fired.

In the table illustrated in FIG. 3 , a content of the glass frit 150G ofthe second electrode 150 is a content in a state where the secondelectrode 150 is fired after the thermal process. An appropriate contentof the glass frit 150G included in the second electrode paste before thethermal process may be 2.0 wt % to 4.0 wt % and may correspond to arange (i.e., 2.5 wt % to 5.0 wt %) of an appropriate content in thetable illustrated in FIG. 3 .

Embodiments of the invention relate to the structure of the solar cell,and thus describes below the structure of the solar cell based on thecontent of the glass frit 150G of the second electrode 150 that is firedafter the thermal process.

In the table illustrated in FIG. 3 , the contact resistance indicates aresistance between the second electrode 150 and the second conductiveregion 170. Thus, the bad contact resistance means that the electricalconnection between the second electrode 150 and the second conductiveregion 170 is not properly performed because the second electrode 150cannot penetrate the back passivation layer 190 as the depth to whichthe second electrode paste is fired through is very thin. Further, thegood contact resistance means that the electrical connection between thesecond electrode 150 and the second conductive region 170 is properlyperformed.

The passivation function (or a recombination level) indicates apassivation function of the control passivation layer 160. Thus, thegood passivation function means that the control passivation layer 160is not damaged. Further, the bad passivation function means that thecontrol passivation layer 160 is damaged by the second electrode 150,and a recombination occurs in the back surface of the semiconductorsubstrate 110 due to the metal particles 150M of the second electrode150.

The good open-circuit voltage Voc means that the second conductiveregion 170 is spaced apart from the semiconductor substrate 110 by thecontrol passivation layer 160, and the solar cell generates theopen-circuit voltage Voc of an appropriate level. The bad open-circuitvoltage Voc means that the control passivation layer 160 is damaged asthe second electrode 150 deeply penetrates into the second conductiveregion 170, and the second electrode 150 and the semiconductor substrate110 are short-circuited.

As indicated by the table of FIG. 3 , when the content of the glass fritper unit volume contained in the second electrode 150 is less than 2.5wt %, the depth to which the second electrode paste is fired through isvery thin. Hence, the second electrode 150 is not properly connected tothe second conductive region 170.

Further, as shown in FIG. 4 , when the content of the glass frit perunit volume contained in the second electrode 150 exceeds 5.0 wt %, thesecond electrode 150 penetrates the control passivation layer 160 aswell as the second conductive region 170 and is short-circuited with thesemiconductor substrate 110. Hence, a function of the controlpassivation layer 160 is damaged, and the open-circuit voltage Voc isdeteriorated.

In FIG. 4 , “150M” denotes the metal particles contained in the secondelectrode 150, and “150G” denotes the glass frit contained in the secondelectrode 150.

When the content of the glass frit per unit volume contained in thesecond electrode 150 maintains an appropriate level of 2.5 wt % to 5.0wt %, the depth to which the second electrode paste is fired through isappropriate. Hence, all of the contact resistance, the passivationfunction, and the open-circuit voltage are maintained at the excellentlevel.

Further, as shown in FIG. 5 , when the content of the glass frit perunit volume contained in the second electrode 150 maintains theappropriate level of 2.5 wt % to 5.0 wt %, the second electrode 150penetrates the back passivation layer 190 and penetrates into the secondconductive region 170 at an appropriate depth. In this instance,alloy-crystallite (hereinafter referred to as “crystallite”) obtained bycombining the metal particles 150M of the second electrode 150 andsilicon of the second conductive region 170 may be formed at aninterface between the second electrode 150 and the second conductiveregion 170.

The metal particles 150M-silicon crystallite (“crystallite”) 153 canfurther reduce the contact resistance between the second electrode 150and the second conductive region 170.

So far, the embodiment of the invention described the content of theglass frit 150G per unit volume included in the second electrode 150, inorder to control the depth to which the second electrode 150 penetratingthe back passivation layer 190 is fired through inside the secondconductive region 170.

The embodiment of the invention describes below an example where theglass frit 150G further includes tellurium oxide (TeO) so that thesecond electrode 150 penetrating the back passivation layer 190 isproperly fired through toward the second conductive region 170 and isconnected to the surface of the second conductive region 170 at anoptimum level.

FIG. 6 is an enlarged cross-sectional view 51 of FIG. 2 of a portion ofthe solar cell including the semiconductor substrate 110, the controlpassivation layer 160, the second conductive region 170, and the secondelectrode 150 when a glass frit further includes tellurium oxide (TeO)in a state where a content of the glass frit of FIGS. 3 and 4 maintainsthe appropriate level.

As described above, the second electrode 150 according to the embodimentof the invention may include the glass frit 150G of 2.5 wt % to 5.0 wt %per unit volume, and the glass frit 150G may include at least one ofPbO-based material or BiO-based material and tellurium oxide (TeO).

A melting point of the glass frit 150G containing tellurium oxide (TeO)may be relatively lower. For example, the melting point of the glassfrit 150G containing tellurium oxide (TeO) may be 200° C. to 500° C.

Thus, the glass frit 150G containing tellurium oxide (TeO) may be firstmelted when the second electrode paste is fired through in the thermalprocess and etches the back passivation layer 190.

In this instance, the glass frit 150G containing tellurium oxide (TeO)may be first widely positioned on the surface of the second conductiveregion 170 to form a layer. Afterwards, the metal particles 150M and theglass frit 150G not containing tellurium oxide (TeO) may be positionedon the layer on which the glass frit 150G containing tellurium oxide(TeO) is positioned.

Hence, as shown in FIG. 6 , the second electrode 150 may include a firstlayer L1 where the glass frit 150G containing tellurium oxide (TeO) ispositioned at an interface between the second electrode 150 and thesecond conductive region 170 and a second layer L2 where the metalparticles 150M and the glass frit 150G not containing tellurium oxide(TeO) is positioned on the first layer L1.

The crystallites 153 formed by combining or a combination of the metalparticles 150M and silicon of the second conductive region 170 may bedistributed at an interface between the first layer L1 and the secondconductive region 170.

Hence, the contact resistance between the second electrode 150 and thesecond conductive region 170 and the open-circuit voltage Voc can befurther improved, and the depth to which the second electrode paste isfired through can be controlled more easily. As a result, a processmargin of the solar cell can be further improved.

So far, the embodiment of the invention described the material and thematerial content of the second electrode 150 capable of controlling thedepth, to which the second electrode paste is fired through, at theappropriate level.

However, the glass frit 150G containing tellurium oxide (TeO) may beapplied to the first electrode 140 as well as the second electrode 150.

For example, a glass frit 150G included in the first electrode 140 mayinclude at least one of PbO-based material or BiO-based material, andthe glass frit 150G may further include tellurium oxide (TeO).

So far, the embodiment of the invention mainly described the glass fritincluded in each of the first and second electrodes 140 and 150.Hereinafter, metal particles included in the first and second electrodes140 and 150 are described in detail.

FIG. 7 illustrates metal particles included in the first and secondelectrodes 140 and 150 according to the embodiment of the invention.

More specifically, FIG. 7 illustrates only metal particles M1 and M2included in the first and second electrodes 140 and 150 and omits theillustration of the glass frit illustrated in FIGS. 5 and 6 forconvenience of explanation.

As shown in FIG. 7 , metal particles M1 and M2 may be included in thefirst and second electrodes 140 and 150.

More specifically, metal particles included in the first electrode 140may include first metal particles M1 each having a sphere shape of acircle or an oval and second metal particles M2, each of which has along axis and has a flake shape of a plate shape having an unevensurface. Further, metal particles included in the second electrode 150may include the first metal particles M1 and may not include the secondmetal particles M2.

For example, as shown in FIG. 7 , the first finger electrode 141 of thefirst electrode 140 may include the first metal particles M1, and thefirst bus bar electrode 142 of the first electrode 140 may include thefirst metal particles M1 and the second metal particles M2.

Further, the second finger electrode 151 and the second bus barelectrode 152 of the second electrode 150 may include the first metalparticles M1 and may not include the second metal particles M2.

A length of the long axis of the second metal particle M2 included inthe first electrode 140 may be greater than a size of the first metalparticle M1 included in each of the first and second electrodes 140 and150.

For example, a diameter of the first metal particle M1 may be 200 nm to2.5 μm, more preferably, 300 nm to 2.0 μm. Further, a size of the secondmetal particle M2 may be 3.0 μm to 6.0 μm.

Hence, the first electrode 140 may include the second metal particles M2having a volume greater than a volume of the second electrode 150.

Because the first electrode 140 includes the second metal particles M2having the relatively large size as described above, reactivity of themetal particles can be further improved in the thermal process forfiring. Further, the electrode can be fired more easily at a relativelylower temperature, and electrical characteristics (for example, aresistance) of the first electrode 140 can be further improved.

The first electrode 140 may be formed by performing a printing processon the first electrode paste twice, and the second electrode 150 may beformed by performing a printing process on the second electrode pasteonce.

More specifically, the first electrode 140 may be formed through firstand second printing processes. In the first printing process, a firstbus bar electrode paste including the first and second metal particlesM1 and M2 may be printed on the front surface of the semiconductorsubstrate 110 in the second direction and then dried. In the secondprinting process, a first finger electrode paste including the firstmetal particles M1 may be printed on the front surface of thesemiconductor substrate 110 in the first direction and then dried. Next,the first electrode 140 may be formed by performing the thermal processon the first bus bar electrode paste and the first finger electrodepaste.

The second electrode 150 may be formed by printing once a secondelectrode paste including the first metal particles M1 on the backsurface of the semiconductor substrate 110 in accordance with a patternof the second finger electrodes 151 and a pattern of the second bus barelectrodes 152 and performing the thermal process on the secondelectrode paste.

However, the metal particles M1 and M2 included in the first and secondelectrodes 140 and 150 are not limited to FIG. 7 and may be formedunlike FIG. 7 .

For example, the first electrode 140 may be formed by printing anddrying a first electrode paste including the first and second metalparticles M1 and M2 in accordance with a pattern of the first fingerelectrodes 141 and a pattern of the first bus bar electrodes 142 andthen printing a separate first electrode paste including only the firstmetal particles M1 in accordance with the pattern of the first fingerelectrodes 141 and the pattern of the first bus bar electrodes 142.

In this instance, the first finger electrodes 141 as well as the firstbus bar electrodes 142 may include the second metal particles M2 havinga relatively larger volume.

Thus, the first electrode 140 may include metal particles having avolume relatively larger than the metal particles of the secondelectrode 150.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the scope of the principles of thisdisclosure. More particularly, various variations and modifications arepossible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

What is claimed is:
 1. A solar cell comprising: a single crystallinesilicon substrate; an emitter region positioned at a front surface ofthe single crystalline silicon substrate; an anti-reflection layerpositioned on the emitter region; a dielectric layer on a back surfaceof the single crystalline silicon substrate; a polysilicon back surfacefield layer on the dielectric layer; a back passivation layer positionedon the polysilicon back surface field layer; a first electrode connectedto the emitter region through the anti-reflection layer; and a secondelectrode connected to the polysilicon back surface field layer throughthe back passivation layer, wherein each of the first and secondelectrodes includes silver particles and a glass frit, whereincrystallites formed by a combination of the silver particles of thesecond electrode and silicon of the polysilicon back surface layer areformed at an interface between the second electrode and the polysiliconback surface layer, but not in either the dielectric layer or the backsurface of the single crystalline silicon substrate, whereincrystallites formed by a combination of the silver particles of thefirst electrode and silicon of a front surface of the emitter region areformed at an interface between the first electrode and the emitterregion, and wherein a content of the glass frit per unit volumecontained in the second electrode is less than a content of the glassfrit per unit volume contained in the first electrode.
 2. The solar cellof claim 1, wherein each of the glass frit of each of the first andsecond electrodes includes tellurium oxide (TeO).
 3. The solar cell ofclaim 2, wherein the second electrode includes: a first layer where theglass frit including tellurium oxide (TeO) is positioned at an interfacebetween the second electrode and the polysilicon back surface fieldlayer; and a second layer where the glass frit not including telluriumoxide (TeO) is positioned on the first layer.
 4. The solar cell of claim1, wherein the content of the glass frit per unit volume contained inthe second electrode is 2.5 wt % to 5.0 wt %.
 5. The solar cell of claim1, wherein a content of the silver particles per unit volume containedin the first electrode is more than a content of the silver particlesper unit volume contained in the second electrode.
 6. The solar cell ofclaim 5, wherein the content of the silver particles per unit volumecontained in the first electrode is 82 wt % to 92 wt %, and wherein thecontent of the silver particles per unit volume contained in the secondelectrode is 68 wt % to 73 wt %.
 7. The solar cell of claim 1, wherein athickness of the back passivation layer is less than a thickness of theanti-reflection layer, wherein the thickness of the anti-reflectionlayer is 100 nm to 140 nm, and wherein the thickness of the backpassivation layer is 65 nm to 105 nm within a range that is less thanthe thickness of the anti-reflection layer.
 8. The solar cell of claim1, wherein the dielectric layer is a control passivation layerpositioned between the back surface of the single crystalline siliconsubstrate and the polysilicon back surface field layer, wherein athickness of the control passivation layer is 0.5 nm to 10 nm.
 9. Thesolar cell of claim 1, wherein a thickness of the emitter region is 300nm to 700 nm, and wherein a thickness of the polysilicon back surfacefield layer is 290 nm to 390 nm within a range that is less than thethickness of the emitter region.
 10. The solar cell of claim 2, whereinthe glass frit of the second electrode further includes at least one ofa PbO-based material or a BiO-based material.
 11. The solar cell ofclaim 2, wherein a melting point of the glass frit including telluriumoxide (TeO) is 200° C. to 500° C.
 12. The solar cell of claim 1, whereinthe glass frit of the first electrode includes at least one of aPbO-based material and a BiO-based material.
 13. The solar cell of claim1, wherein the silver particles of the first electrode include firstsilver particles having a circular shape or an oval shape and secondsilver particles which have a long axis and have a plate shape having anuneven surface, and wherein the silver particles of the second electrodeinclude the first silver particles and do not include the second silverparticles.
 14. The solar cell of claim 13, wherein a length of the longaxis of the second silver particle included in the first electrode isgreater than a size of the first silver particle included in each of thefirst and second electrodes.
 15. The solar cell of claim 10, wherein theglass frit of the second electrode includes the PbO-based material. 16.The solar cell of claim 12, wherein the glass frit of the firstelectrode includes the PbO-based material.
 17. The solar cell of claim1, wherein the interface between the second electrode and thepolysilicon back surface layer is located between the back surface ofthe polysilicon back surface layer and an interior of polysilicon backsurface layer.
 18. The solar cell of claim 3, wherein the second layerhas a thickness that is less than a thickness of the back passivationlayer.
 19. The solar cell of claim 4, wherein the content of the glassfrit per unit volume contained in the first electrode is 6 wt % to 8 wt%.
 20. The solar cell of claim 1, wherein the emitter region ispositioned directly on the front surface of the single crystallinesilicon substrate and is formed from a same single crystalline siliconmaterial from which the single crystalline silicon substrate is formed.